This invention relates to a method by which the radioactive contamination profile of pipes that are embedded within concrete structures in the premises of a nuclear reactor facility and which have been contaminated radioactively during the operation of the facility can be evaluated through measurement of .beta.-rays. The method is applicable not only to the measurement and evaluation of the radioactive contamination profile of pipes embedded in a nuclear reactor facility scheduled to be decommissioned but also to other areas of radiation measurement and evaluation of the inner surfaces of pipes embedded in concrete structures including those of RI (radioisotope) utilizing facilities.
The inner surfaces of pipes in nuclear reactor facilities are contaminated radioactively by nuclides such as .sup.60 Co, .sup.63 Ni and .sup.137 Cs originating from nuclear fission or from the corrosion products formed as the result of circulation of primary coolants and the like in the nuclear reactor facilities. Most of the contaminating nuclides emit .beta.- and .gamma.-rays. In order to measure the radioactive contamination profile of the inner surfaces of the contaminated pipes, a .beta.- or .gamma.-ray detector is inserted into the pipe of interest and the intensity of its radioactivity is measured. To meet this need, the detector must be small and yet have high sensitivity.
In the conventional measurement of the radioactive contamination profile of the inner surfaces of contaminated pipes, a germanium semiconductor detector, a sodium iodide type detector and the like are used to measure .gamma.-rays. However, the germanium semiconductor detector requires the cooling of the solid-state radiation probe with liquid nitrogen and, in addition, the great penetrating power of .gamma.-rays requires the provision of a collimator which limits the surface of measurement; these have been an obstacle to the effort to reduce the size of the detector. Similarly, the sodium iodide type detector requires the provision of a collimator for limiting the surface of measurement and this has also been an obstacle to the effort to realize size reduction.
Another approach that could be taken for radiation measurement is by cutting a sample of the pipe of interest but this method is not practically feasible if the pipe is embedded in a concrete structure.